Avoiding Interrupt Request and Communication Port Conflicts

Each serial device attached to your computer needs its own unique communication port(COM Port) and interrupt request line(IRQ). Your internal modem, assuming it's not in Plug and Play mode, may be preset to a COM Port and IRQ, which is already in use by your system. If your modem is in Plug and Play mode, it is configured through software during installation. If any other device is using the same COM Port or IRQ as your modem, it will cause a conflict. This can result in system lock-ups or loss of data.

Common Sources of COM Port and IRQ Conflicts
Serial interface cards or external COM Ports, are the most common source of COM Port or IRQ conflicts. There are many other devices in your system that can cause conflicts as well. Soundcards frequently cause conflicts since they can use up to three IRQs, depending on the card. Other devices can conflict with your modem, such as SCSI cards, pre-installed modems, or other peripherals in your system.

External COM Ports
Most computers come with two external COM Ports (COM1 and COM2). If you have an external COM2 connector (look on the back of your computer), you will have a conflict if you install an internal modem using COM2. Even if nothing is connected to the external port, the COM Port is still reserved for that Port, unless you disable it in the system BIOS(see below). The reason this happens is that the external connector uses a serial interface card, which reserves COM2 and IRQ3 for the external connector.

Disabling The External COM2 Connector
If you do not plan to use the external COM2 connector, most computers will let you disable it (consult your computer’s user guide or its manufacturer’s technical support). Your internal modem may then be installed using COM2 and IRQ3, if you choose. Modems are not required to be on COM2 and IRQ3, but we are using this as an example.

Serial Interface/Expansion Cards
Sound cards, network cards, SCSI/IDE, and other serial interface cards use COM Ports and IRQs. You may have installed such a card in your computer’s expansion slot to run an external hard drive, or CD-ROM, or perhaps to connect to your network. Some video cards also use an IRQ. The main issue to watch for with video cards is that a lot of them share COM4’s memory address space. This can conflict if you try putting a modem on COM4. Please contact your graphic card company for support on this issue.

You need to determine which COM Port and IRQ each serial interface card in your system is using. One way to find this out is to open up your computer, take out the expansion card or cards, and examine their jumper settings (be sure the computer is turned off before removing an expansion card). Another simpler way is to use Microsoft’s Diagnostics program.

Using Microsoft Diagnostics
Microsoft Diagnostics is a program included with most versions of MS-DOS. This program tells you what is installed on each COM Port and IRQ. Simply type MSD at the DOS prompt. If you are in Windows 3.1 or Windows for Workgroups, exit Windows completely, then run MSD. If you are running Windows NT, click on Run, then type WINMSD. For Windows 95 users, you can drop to a DOS shell and type MSD. Look under “IRQ Status” and “COM Ports”. If COM3 or COM4 is available, it will be labeled “N/A”. We do not recommend COM4, due to the potential graphics card conflict mentioned above.

IRQ Shuffling
Sometimes it is necessary to change the IRQ setting on an expansion card to free an IRQ for the modem. For example, if your sound card is using IRQ5, you can reinstall it using IRQ11. This would then free IRQ5 for use by your modem. Note that most serial devices, including your modem, are incapable of operating at IRQs above 7.

Determining a COM Port and IRQ to use for your Modem
Your modem can be set to the following IRQs:

IRQ 2, 3, 4, 5, or 7

The IRQs available for your modem to use, depends on your computer setup. Each IRQ on your computer is assigned a “peripheral device” or other function(you can consult your computer documentation to see what these default settings are, or use MSD, as explained above). The typical assignment for the first eleven IRQs are listed below:

IRQ Assignment

0 System Timer

1 Keyboard

2 Cascade input(mandatory function)

3 COM2

4 COM1

5 Parallel Port 2 (LPT2)

6 Disk Controller (All internal floppy drives)

7 Parallel Port 1 (LPT1)

8 Real-time clock interrupt

9 Reserved

10 Reserved

11 Reserved

As you can see above, there are no default assignments for COM3 and COM4. In order to use COM3 or COM4, you must assign it to an IRQ. This means “sharing” that IRQ with it’s default assignment. This is possible, ONLY if the default assignment is NOT in use. For example, you can assign COM3 to IRQ5, if you are not using LPT2 for a second printer, or other external parallel device. You CAN NOT assign COM4 to IRQ2 because IRQ2 is being used by the computer for a mandatory function. If you have a mouse attached to COM1, IRQ4 is in use by COM1 and can not be used for COM3. If you plan to use COM3 or COM4, you must assign these COM Ports to an IRQ through Windows or your DOS communications software. Before running your software, you must first re-install your modem with the right IRQ and COM Port settings(either jumpers or software set for plug and play devices).

Re-Installing your Modem
Begin by turning off your computer. Open up your computer to reveal the expansion slots where your internal modem was originally installed. Remove your internal modem.

To change the COM Port and IRQ settings, you need to configure the jumpers on the board. If you have one of our plug and play modems, the jumpers should be removed, if you particular unit has jumpers. Plug and Play modems are configured through software. If you do not have a plug and play modem or plug and play software, you will have to use the jumpers to configure the modem. Each jumper on the modem board has two upright metal pins connected by a black plastic piece called a shunt. This shunt closes the circuit between the two pins. Depending on your particular modem, a picture of your modem’s jumper settings are either on the opposite side of the board, or in your manual. To change the settings, gently lift off the black shunt and replace it over the contacts next to the appropriate COM or IRQ location. Replace the modem in the expansion slot, close the cover, and turn on your computer.

Running your Communications Software
After you have re-installed the modem with the correct COM and IRQ settings, you must change the settings in your software to match the modem.

Windows Applications
If your communication program is running under Microsoft Windows, you must first assign the modem’s COM Port to the IRQ you selected through the Window’s Control Panel. Use the following steps:

• In the Main menu of the Program Manager, open up the Control Panel.
• Double-click on the Ports icon.
• Use the mouse to select the COM Port that you are using for your modem.
• Click on Settings..., then, Advanced
• Select the IRQ you want to use for your modem.

Be sure to select the appropriate COM Port in your communications software. Consult your software’s documentation for information on how to make these changes.

DOS Applications
If you are running a DOS communication program, you need to change the COM and IRQ settings in your communication software. Consult your software’s documentation for more information on how to make these changes.

Technical Support
If you continue to have conflicts, or do not know how to disable the on-board COM Ports, or other settings, you should contact your computer manufacturer or software manufacturer. If you are sure that there are no conflicts in your system, and your USRobotics modem still does not function, please contact our technical support department for further assistance.